Safety Moment #34: The Purloined Letter

Purloined letter and process safety

One of the world’s first detective stories, written in the year 1844, was Edgar Allen Poe’s The Purloined Letter set in the city of Paris. In that story the bad guy — Minister D — has in his possession a letter containing compromising information about an unnamed lady. He is using this information to blackmail her. There is only one copy of the letter (reminder: there were no copying machines and scanners in those days).

No worries — the Paris Prefect of Police is on the case. He and his and his men have searched Minister D’s residence thoroughly, even using a magnifying glass to examine the tables and chairs. But they have found nothing. So they bring in one of the first famous amateur detectives: C. Auguste Dupin. He quickly finds the letter — it is hidden in plain view on a card rack in D’s room.

I was put in mind of this story when I recalled a situation that occurred early in my career. We had just started an ethylene oxide (EO) plant with a rated production capacity of 100 tons/day. After the usual start-up hiccups we got the plant up and running with process conditions pretty much where they should be. In particular, the temperatures, pressures and flow rates in the EO reactor were at design values.

But the production rate of EO wasn’t 100 tons/day, it was 90 tons. Where, we asked, was the missing 10 tons?

The sketch below provides a high-level view of the process.

Ethylene oxide production example of overlooked issue

Ethylene and oxygen are mixed in the reactor to form an EO vapor stream that also contains unreacted ethylene and various impurities. This crude EO stream flows into an Absorber, down which flows cool water. The EO dissolves in the water; the unreacted gases leave the top of the Absorber and are returned to the Reactor. The rich water from the bottom of the Absorber is sent to a Stripper into which live steam is added. Purified EO vapor leaves the top of the Stripper, the lean water is recycled back to the Absorber.

The lean water leaving the Stripper is cooled in a Cooling Tower before it enters the Absorber. A hydrocarbon detector is placed in the air stream leaving the top of the tower.

During the start-up the hydrocarbon detector indicated that there were flammable materials in the water vapor leaving the cooling tower. The alarm was ignored. The instrumentation on the facility was not of high quality, so the alarm, in as much as anyone paid attention to it, was treated as “instrument error”.

In fact, the “instrument error” signal was an example of Poe’s purloined letter. It was telling us the reason for the production losses. But we didn’t see it because it was hidden in plain view.

What had happened was that the temperature in the base of the Stripper was slightly too low. Hence the lean water leaving the bottom of the Stripper contained some dissolved EO. When this stream flowed through the cooling tower the EO was vaporized and was literally blown into the sky and the hydrocarbon alarm was triggered.

Having figured out the problem the steam flow rate was increased by a small amount; the temperature at the base of the Stripper went up and — about 15 minutes later — the flow rate of EO product to the storage tanks jumped from 90 to 100 tons/day. And the cooling tower alarm stopped sending a warning signal.

The failure to treat a warning signal seriously was only one of the issues to do with this event — not even the most important. Nevertheless, it is a reminder to always keep an eye out for the purloined letter that is hidden in plain view, i.e., to pay attention to all warning signs and “must be wrong” instrument signals, no matter how unlikely or implausible they may be.